1. Field of the Invention
The present invention relates to a method and apparatus for converting a data rate including, for example, converting frame rates of video signals under different systems.
2. Description of the Related Art
The NTSC system and PAL system are widely used video signal systems. A frame rate of video signals under the NTSC system and a frame rate of video signals under the PAL system are different. Herein, a xe2x80x9cframe ratexe2x80x9d refers to a time period required for displaying one screen.
The frame rate under the NTSC system is about 30 frames/second ({fraction (1/30)} second cycle). (Precisely, the frame rate is 30xc3x971000/1001 frames/second.) The frame rate under the PAL system is 25 frames/second ({fraction (1/25)} second cycle). Therefore, when video signals under the NTSC system are attempted to be displayed under the PAL system, or when video signals under the PAL system are attempted to be displayed under the NTSC system, the frame rate should be converted.
As a conventional method for converting a data rate, Japanese Laid-Open Publication No. 8-172573 discloses a method for converting low rate data frames to high rate data fields, where 1 frame=2 fields, and 1 screen=1 frame.
In Japanese Laid-Open Publication No. 8-172573, the data rate is converted through the following steps. A basic field repetition rate such that fields are repeated a number (i.e., integer) of times during one frame period is determined based on an acceleration ratio i.e., a ratio of a high rate to a low rate. Then, a differential of a ratio between repetition rates is calculated by subtracting a ratio of the number of fields in one frame period to the low rate from the acceleration ratio. A differential of the field repetition rate and the differential of the ratio are compared; if they vary with respect to each other, a field is added or deleted to convert the data rate.
In accordance with the prior art described above, a frame rate of video signals under the PAL system (25 frames/second) is converted to a field rate of video signals under the NTSC system (about 60 fields/second) in the following manner. By calculating the repetition rate with the method according to the prior art described above, based on an acceleration ratio, i.e., a ratio of the data rate under the PAL system and the data rate under the NTSC system (60/25=2.4), the repetition rate is determined to be 2 fields and 3 fields. A differential calculation of the acceleration ratio is performed, and the number of fields to be repeated to account for the variation is calculated to be 2, 3, 2, 2, and 3 with the method according to the prior art described above.
FIG. 5 is a timing diagram showing the timing to perform the conversion of the data rate from the PAL system to the NTSC system. Based on the number of the fields to be repeated, 2, 3, 2, 2, 3, the data rate can be converted by outputting frames H1, H1, H2, H2, H2, H3, H3, H4, H4, H5, H5, H5 for the input frames H1 to H5.
However, in accordance with the prior art, the information having a low data rate (video signals under the PAL system, {fraction (1/25)} second cycle) can be converted to information having a higher data rate (video signals under the NTSC system, {fraction (1/30)} second cycle), but information having a high data rate cannot be converted to information having a lower data rate.
Also, as indicated by start time T1 of the input frame H1 in FIG. 5, the input frames (frames under the PAL system) and the output frames (frames under the NTSC system) should be synchronous.
According to one aspect of this invention, there is provided a data rate conversion apparatus, comprising: an information input device for receiving frame information at a first data rate; an information storage memory including a plurality of buffers for storing the frame information; a write control device for selecting one of the buffers to which the frame information is to be written, and writing the frame information to the selected buffer; a read control device for selecting one of the buffers from which the frame information is to be read, and reading the frame information from the selected buffer; and an information output device for outputting the frame information, which is read by the read control device, at a second data rate which is different from the first data rate.
In one embodiment of the invention, the data rate conversion apparatus further comprises an input termination detection input device for outputting a frame information termination signal indicating a termination of an input of the frame information to the information input device, wherein the write control device selects one of the buffers to which frame information is to be input next in response to the frame information input termination signal, and holds a first selected buffer information indicating the selected buffer.
In one embodiment of the invention, the write control device selects one of the buffers for a buffer to which frame information to be input next is to be written, the buffer being other than the buffer to which frame information, which has been input to the information input device, is written and the buffer from which the read control device is reading the frame information.
In one embodiment of the invention, the read control device selects one of the buffers from which frame information to be output next is read, and holds a second selected buffer information indicating the selected buffer.
In one embodiment of the invention, the read control device selects the buffer to which the frame information, which has been input to the information input device, is written for a buffer from which next output frame information is to be read.
In one embodiment of the invention, the data rate conversion apparatus further comprises an input synchronization detection device for outputting a first synchronization signal corresponding to the first data rate, wherein the write control device writes frame information to be input next to the buffer indicated by the first selected buffer information, in response to the first synchronization signal.
In one embodiment of the invention, the data rate conversion apparatus further comprises an output synchronization detection device for outputting a second synchronization signal corresponding the second data rate, wherein the read control device reads frame information to be output next from the buffer indicated by the second selected buffer information, in response to the second synchronization signal.
In one embodiment of the invention, the first synchronization signal includes a signal indicating a timing to start an input of the frame information to be input next.
In one embodiment of the invention, the second synchronization signal includes a signal indicating a timing to start an output of the frame information to be output next.
In one embodiment of the invention, the information storage memory includes three buffers.
According to one aspect of the present invention, at the time when an input of information of one frame from the information input device terminates, a buffer to which the next input information is to be written and a buffer from which the next output information is to be read are selected. The write control device writes the information to the selected buffer at the next input frame start time, and the read control device reads the information of the selected buffer at the next output frame start time.
Thus, the invention described herein makes possible the advantages of (1) providing an apparatus which, with a single structure, can convert input information having an arbitrary data rate to output information having another arbitrary data rate, whether converting from a lower rate to a higher rate or vice versa; and (2) providing an apparatus with which it is possible to convert data rates even when input information and output information are asynchronous.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.